Evidence of Host Association and Geographic Structure Among Historical Strains and Modern Populations of Sclerotinia homoeocarpa.

Dollar spot is the most economically important disease of turfgrass worldwide, but how evolutionary forces have shaped the global population biology of its causal organism, Sclerotinia homoeocarpa, is not well understood.  The objective of this research is to infer the population structure of S. homoeocarpa on a global scale.  We performed genotyping at 14 microsatellite loci and determined mating type with a multiplex PCR assay.  A total of 694 S. homoeocarpa isolates obtained from 54 locations on five continents and Oceania have been evaluated to date.  In addition to previously known structure between C3 and C4 host type isolates, Bayesian clustering analysis revealed limited admixture, substructure within each group, and a third group comprised of a sample from Puerto Rico that did not associate with isolates from either C3 or C4 grasses.  Within each type, some isolates assigned to the same group, and in select cases belonging to identical multilocus haplotypes, originated from multiple continents.  Within all but one of 125 multilocus haplotypes, all isolates were of the same mating type.  Clone correction showed that the MAT1-1:MAT1-2 distribution was approximately 2:1 and 1:2 inside and outside North America, respectively, but this deviation was significantly different from 1:1 only for outside North America.  Among several locations in the United Kingdom evaluated to date, microsatellite genotyping failed for several isolates from one location that exhibited disorganized culture morphology.  All isolates from this location, and from a second location that showed similar morphology, were subjected to ITS sequence analysis.  Although collected in 2008, the ITS sequences indicated that these populations are divergent from modern strains and identical to culture specimens submitted by F.T. Bennett and N. Jackson in 1937 and 1973, respectively.   The culture specimens and the two populations sampled in 2008 were obtained from fine leaf fescues (Festuca sp.) in the United Kingdom managed under similar conditions.  Initial results from this research raise the possibility that multiple fungi can cause disease symptoms resembling dollar spot, and suggest that the modern Sclerotinia homoeocarpa has undergone long distance dissemination, sexual reproduction, and clonal amplification. 

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